1. Field of the Invention
The present invention relates to a buffer managing system in an ATM (asynchronous transfer mode) node equipment having a buffer memory for temporarily storing ATM cells. More specifically, the invention relates to a buffer managing system which can improve an effective throughput of upper layer packet level with a least buffer memory.
2. Description of the Related Art
In an ATM node equipment, such as an ATM switch, a buffer memory for avoiding interference of ATM cells directed to the same output line is employed. The capacity of the buffer memory is finite. When inputs exceeding allowable range are made, the buffer memory should cause overflow to dispose the overflown ATM cells. The upper layer packet having even one drop out cell cannot be utilized by upper protocol layer. Therefore, even when such packet reaches a destination terminal, it is instantly abandoned and re-transmission process is taken place.
It is not at all desirable to accumulate and transmit such packet including lost cell knowing as being useless in the light of effective use of a net work resource. This can be a cause of traffic congestion.
In order to avoid wasting of the net work resource by such useless cells, a selective packet disposing method, in which if a buffer occupied amount at a timing of arrival of a leading cell of an AAL (ATM adaptation larger) packet exceeds a predetermined threshold value, all of the cells from the leading cell to a trailing end cell (EOM (End of Message) cell) forming the AAL packet are abandoned before buffering.
In this selective packet disposing method, a packet disposing management table for managing whether packet is in reception or in disposal per logical channel (VC), becomes necessary. However, in the conventional selective packet disposing method, states provided for one logical channel (VC) are "packet not received" state, "packet in reception" state and "packet in disposal" state.
In this selective packet disposing method, when the EOM cell arrives via the currently used logical channel (VC), the state in the table is set at "packet not received". When the cell other than EOM cell arrives from the logical channel (VC) in the "packet not received" state, the occupied amount of the entire buffer is checked at this timing. If the occupied amount is in excess of the threshold value, the "packet in disposal" state is set in order to forcedly dispose all of the cells arriving through the current logical channel (VC) until the EOM cell arrives.
If the occupied amount is less than or equal to the threshold value, the "packet in reception" state is set to continuously receive the packet until the next EOM cell arrives. A flowchart of the conventional selective packet disposal is shown in FIG. 16.
Next, the operation of the conventional method will be discussed with reference to the flowchart of FIG. 16. At first, when the ATM cell is input to a cell input control portion (G1), the cell disposal managing table is made reference to on the basis of a logical channel identifier (VCI) (G2), and then an attribute registered in the management table is checked (G3). If the attribute is "10" ("packet reception enable" state), the ATM cell is accumulated in the cell buffer as it is (G4). Finally, check is performed whether the arrived cell is the EOM cell or not (G5). If the arrived cell is EOM cell, the state of the corresponding logical channel (VC) is modified to be "01" ("packet not received" state) (G6).
On the other hand, when the attribute as checked at step G3 is "11" ("packet in disposal" state), the ATM cell is forcedly dropped in the cell input control portion (G7). In step G3, if the state of the logical channel (VC) is "01" ("packet not received" state), the arriving ATM cell may be regarded as the leading cell of the upper layer packet.
Therefore, next, the occupied amount of the entire cell buffer is measured at this timing (G8). If the occupied amount Q.sub.-- total of the entire buffer is less than or equal to the predetermined through value Qth.sub.-- total (G9), judgement is made that all of the ATM cells of the currently arriving packet may be stored to set the state of the logical channel (VC) at "10" ("packet reception enable" state) (G10). On the other hand, when the occupied amount Q.sub.-- total of the entire buffer is greater than or equal to the threshold value Qth.sub.-- total (G9), the state of the logical channel is set at "11" ("packet in disposal" state) is set (G11) to drop the ATM cells.
In the conventional method as set forth above, selective packet disposal is uniformly performed with respect to all of the logical channels (VC). However, there are some logical channels which has demand to be transmitted the cells as much as possible irrespective of loss of some cells. For such type of logical channel (VC), it is not desirable to effect the selective packet disposal.
Also, in the conventional method, since judgement whether the arriving packet is to be received or disposed is made on the basis of the occupied amount of the entire buffer and the threshold value, it is inclined to packet-by-packet control irrespective of nature of respective of logical channels (VC). Therefore, under the environment where logical channels (VC) having different traffic characteristics are admixed, it is possible to frequency of forced disposal of packet tends to be deviated per logical channels.